This lab is an introduction to the construction of a standard compensated voltage probe.
Correct Probe Compensation
Due
to inherent inherent non-uniformities in the materials used to
construct compensated voltage probes, they must be manually adjusted,
or compensated, for each piece of test equipment they are connected to.
This is done with an adjustment on either the probe tip or
connector. For the Tectronix scopes used in this lab, the probe
type is set in each channel's menu. Once the type is set , the
probe can be compensated using the probe test function and using the
adjustment to obtain a correct reading. Here are a few examples
of what a compensation reading looks like:
An under-compensated probe:
A correctly compensated probe:
An over-compensated probe:
To
understand what is happening with compensation, it is necessary to know
what the basic construction of a compensated probe looks like.
Below is an example of a basic 10:1 probe schematic: Showing the AC Analysis for the V at the probe tip and the V seen by the scope.
Vprobe-tip to Vscope-in
is 1:0.1 or 10:1. It is worth noting that at 1 VAC the current the
circuit being tested must supply to the scope is 0.1 uA + j67.86 uA,
which may significantly impact the circuits performance.
Cable Capacitance
Coaxial
cables have a built in capacitance due to their construction that is
proportional to their length. This can be measured with a
capacitance meter, or we can use a test setup with the scope and
compensated probe. The pictures below show the readings from each method for the same cable.
Scope measurement
Meter measurement
The scope measurement calculates the capacitance by tan(ɸ) = 1/RjωC => C = 1/Rjωtan(ɸ). For this cable the scope calculated .213nF vs .131nF for the meter. While these measurements are off, it may be due to connection effects from the different setups.
Probe effects on measurements
The
two plots below are measurements of the same circuit taken with a
strait cable and a compensated probe. There is a large
difference, ~6.5x in the output (ch3 vs ch2), due to the load each
measurement device put on the circuit. This is one of the great
benefits to using the compensated probe. Ideally a 10x probe will
have 10x the impedance as a strait cable. Real world factors
reduce this (to ~6.5x in this case)
but the improvement is still significant. The impedance can be
improved further using larger scale probes if needed (ie. 100x).
Measurement with a strait cable
Measurement with a compensated probe
Circuit Board Test Point
Finally,
a test point may be instituted on a PCB so that a compensated probe is
not needed to test the circuit. Essentially an interface is
created so that testing the circuit has no noticable effects on the
circuit operation. This is because the resistor and capacitor
from the compensated probe are already included in the circuit design
so thier effects are already included in the normal operation.
